The present invention relates to sensors for detecting the level of liquid in a container or chamber. More particularly, the present invention relates to inductive liquid level sensors of the type which utilize bifilar wound coils, one of which serves as the primary coil of a sensing system and the other of which is the pick-up or secondary coil for detecting the liquid level. Such inductive liquid level sensors typically utilize shielded cables which are wound side-by-side about a conductive tubular support with the grounded sheath of the cables being brazed to the support tube. This coil wound support tube is then inserted into a sealed thimble, which is then immersible into the liquid in the chamber in which the level is to be sensed. Such coils are typically used to measure liquid metal levels, such as sodium or potassium in a breeder reactor. A constant current is driven through the primary coil and the signal induced in the secondary coil is a function of the shunting of the secondary by the conductive liquid metal.
In the manufacture of such liquid metal level probes, the coil wound tube may extend for many feet which must then be insertable into the tubular thimble. It has been the practice to utilize a cap-like end support which fits over the extending end of the tubular support upon which the coils are wound. The end support acts as a guide during insertion of the coil wound tube into the thimble. Such prior art liquid level sensors have suffered from a lack of signal linearity at the extending ends of the sensor probe. This means that it is very difficult to determine whether the liquid metal containing chamber is completely empty at the bottom end of the chamber or completely full at the top of the chamber. This can have important safety ramifications because of the highly reactive nature of the liquid metals.
The cap-shaped end support members are relatively massive and are brazed to the exterior surface of the coil supporting tube so that the coils are spaced some distance from the actual end of the probe. The end supports effectively support the coil wound tube within the thimble in a rigid fashion.
It is desirable to be able to extend the range of signal linearity for the full length of the probe. This must be done while maintaining an effective end support for the coil wound tube within the thimble.